The subject matter disclosed herein relates to turbine airfoil design.
Traditional turbine blade designs use an arcuate camber line whose radius of curvature varies continuously from leading edge to trailing edge but is always of one sign such that it is purely concave. Further, the thickness distribution along the camber line for traditional gas turbine blades is also arcuate with a radius of curvature that varies continuously from leading edge to trailing edge but is always of one sign such that it is also purely concave. Such configurations lead to energy extraction and relatively efficient flow through the turbine when gas flow is two dimensional in the plane defined by the camber line in a cylindrical polar coordinate frame.
The flow has often been observed to be substantially three dimensional and out of plane and, in these cases, the pure concavity of turbine blades can be less efficient than the two dimensional case. Thus, the desire for increased turbine blade efficiency where the flow is three dimensional has driven traditional airfoil shapes toward thin trailing edges, customized camber lines for aft loading and spanwise leaning and bowing to impose radial pressure gradients to modulate the distribution of flow through the passage.
Often, however, mechanical constraints limit trailing edge thinness and the rotation of blades requires the use of radial blade elements to avoid high bending loads during rotation, which precludes aggressive bowing and leaning. In view of these outcomes, endwall contouring with bumps and gouges within the blade passage and extensions up and downstream have been described to modulate the secondary flow development in the neighborhood of the blade root endwall. Unfortunately, endwall contouring can lead to manufacturing and implementation challenges like casting the gouges or the need for a wavy under platform friction damper for rotor blades.